What term describes soils that are undesirable for a solid foundation due to change with moisture?

The term that best describes soils that are undesirable for a solid foundation due to their change with moisture is "expansion." Soils that expand and contract significantly with moisture changes can create unstable conditions for structures built on them. This behavior is especially prevalent in clay soils, which can swell when wet and shrink when dry.

As the moisture content fluctuates, these types of soils can exert considerable stress on foundations, leading to potential structural issues such as cracking or settling. For a solid foundation, soil stability is crucial; therefore, the ability of soil to expand and contract makes it less desirable for construction purposes.

While plasticity refers to the ability of soil to undergo deformation without breaking, it is not specifically tied to instability arising from moisture changes. Similarly, contraction is a change in volume upon drying but does not encompass the full spectrum of issues related to moisture dynamics. Cohesion describes the attraction between soil particles, which can influence stability but does not inherently relate to moisture fluctuations and their impact on solid foundations. Consequently, the characteristics of expansion directly tie to the changes in soil conditions due to moisture, making it the correct choice for this question.